Hydraulic trailer brake circuit for adjustable gain and improved stability

ABSTRACT

A braking system of a work machine includes a pressurized fluid supply, a base valve fluidly coupled to the supply, a proportional control valve fluidly coupled to the supply, and a fluid output configured to be fluidly coupled to a braking system of a trailer. A first fluid path is selectively fluidly coupling the supply to the fluid output via the base valve, and a second fluid path is selectively fluidly coupling the supply to the fluid output via the proportional control valve. The base valve includes a predefined fixed gain and the proportional control valve includes an adjustable ratio.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/222,097, filed Jul. 28, 2016, the disclosure of which ishereby incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a trailer hydraulic brake controlcircuit, and in particular, to a hydraulic brake control circuit withadjustable gain and increased stability.

BACKGROUND OF THE DISCLOSURE

Tractor trailer combinations generally require a braking system on thetrailer that is engaged when a braking system of the tractor isselectively engaged. Often the braking system of the trailer is coupledto the braking system of the tractor to allow the tractor tosimultaneously apply the braking system of the tractor and the trailer.Current trailer brake control circuits typically utilize a singletrailer brake valve with a fixed hydraulic gain. When the brakes of thetractor are applied, a pilot pressure controls the single trailer brakevalve. The single trailer brake valve provides hydraulic fluid andpressure to the braking system of the trailer under fixed gainconditions.

SUMMARY

In one embodiment of the present disclosure, a braking system of a workmachine includes a pressurized fluid supply; a base valve fluidlycoupled to the supply; a proportional control valve fluidly coupled tothe supply; a fluid output configured to be fluidly coupled to a brakingsystem of a trailer; a first fluid path selectively fluidly coupling thesupply to the fluid output via the base valve; a second fluid pathselectively fluidly coupling the supply to the fluid output via theproportional control valve; wherein, the base valve comprises apredefined fixed gain and the proportional control valve comprises anadjustable gain.

In a first example of this embodiment, the braking system includes ashuttle valve fluidly coupled between the base valve, the proportionalcontrol valve, and the fluid output, where the shuttle valve is movablein response to a difference between a first pressure in the first flowpath and a second pressure the second flow path. In a second example, asolenoid valve is disposed in fluid communication between theproportional control valve and the fluid output, where the solenoidvalve is movable between an open position in which the proportionalcontrol valve is fluidly coupled to the fluid output, and a closedposition in which the proportional control valve is not fluidly coupledto the fluid output. In a third example, the braking system includes acontroller; and a solenoid of the solenoid valve disposed in electricalcommunication with the controller, where movement of the solenoid valvebetween its open and closed positions is controlled by the controller.

In a fourth example, the braking system includes a controller; a servicebrake valve in fluid communication with the base valve, the servicebrake valve configured to provide fluid upon actuation of a servicebrake of the work machine; a sensor disposed in electrical communicationwith the controller, the sensor configured to detect a brake pressurefrom the service brake valve; wherein, the proportional control valve isoperably controlled by the controller to provide an output gain as afunction of the brake pressure. In a fifth example, the controllerreceives a valve set ratio and operably controls an output of theproportional control valve as a function of the brake pressure and valveset ratio. In a further example, the controller operably controls theproportional control valve according to a variable current that isproportional to the brake pressure.

In another embodiment of this disclosure, a method of controllingpressurized fluid to a trailer brake system includes providing acontroller, a fluid supply, a work machine brake system, a brake outputin fluid communication with the trailer brake system, a base valvehaving a preset fixed gain fluidly coupled between the fluid supply andthe brake output, a proportional control valve fluidly coupled to thefluid supply, a solenoid valve, and a shuttle valve; fluidly couplingthe work machine brake system to the base valve via a pilot line;generating a base valve output pressure based on a fluid pressure in thepilot line and the preset fixed gain; identifying, with the controller,a valve set ratio for the proportional control valve; detecting a brakepressure in the work machine brake system with a sensor; communicatingthe brake pressure to the controller via the sensor; determining a brakecommand by the controller as a function of the brake pressure and thevalve set ratio; controlling an output pressure of the proportionalcontrol valve with the controller as a function of the valve set ratioand the brake command; and producing fluid pressure to the brake outputbased on the greater of the output pressure of the proportional controlvalve and the base valve output pressure.

In one example of this embodiment, the method may include providing afirst brake and a second brake of the work machine brake system, thefirst brake generating a first brake pressure and the second brakegenerating a second brake pressure; fluidly coupling a first sensor tothe first brake for detecting the first brake pressure and a secondsensor to the second brake for detecting the second brake pressure,wherein the first sensor and the second sensor are electrically coupledto the controller; wherein, the determining step comprises calculatingthe brake command as a function of the first brake pressure, the secondbrake pressure, and the valve set ratio. In a second example, thedetermining step includes multiplying the valve set ratio by an averageof the first and second brake pressures. In a third example, thedetermining step comprises multiplying the valve set ratio by either thehigher or lower of the first and second brake pressures.

In a fourth example, the controlling step includes calculating the brakecommand as a function of the first brake pressure, the second brakepressure, and the valve set ratio; comparing the brake command to thevalve set ratio; and outputting a current to the proportional controlvalve based on the greater of the brake command and the valve set ratio.In a fifth example, the controlling step includes determining an amountof current in proportion to the detected brake pressure; and sending theamount of current by the controller to the proportional control valve;wherein, the amount of current is adjustable based on the valve setratio. In a sixth example, the method may include providing the solenoidvalve in fluid communication between the proportional control valve andthe fluid output, where the solenoid valve includes a solenoid disposedin electrical communication with the controller; and operablycontrolling movement of the solenoid valve between an open position anda closed position, wherein in the open position the output pressure ofthe proportional control valve is fluidly coupled to the brake output,and in the closed position the output pressure of the proportionalcontrol valve is not fluidly coupled to the brake output. In anotherexample, the producing step comprises actuating the shuttle valve basedon the greater of the output pressure of the proportional control valveand the base valve output pressure.

In a further embodiment of this disclosure, a braking system of atractor trailer combination includes a pressurized fluid supply; a firstbrake and a second brake of the tractor, the first brake providing afirst brake pressure and the second brake providing a second brakepressure; a shuttle valve disposed in fluid communication with the firstbrake pressure and the second brake pressure; a base valve fluidlycoupled to the supply, the base valve including an inlet and an outlet,where the inlet is fluidly coupled to the first shuttle valve forreceiving the greater of the first brake pressure and the second brakepressure; a proportional control valve fluidly coupled to the supply; afluid output configured to be fluidly coupled to a braking system of atrailer; a first fluid path selectively fluidly coupling the supply tothe fluid output via the base valve; and a second fluid path selectivelyfluidly coupling the supply to the fluid output via the proportionalcontrol valve; wherein, the base valve comprises a fixed gain and theproportional control valve comprises an adjustable gain.

In one example of this embodiment, the system may include a secondshuttle valve fluidly coupled between the base valve, the proportionalcontrol valve, and the fluid output, where the shuttle valve is movablein response to a difference between a first pressure output by the basevalve and a second pressure output by the proportional control valve. Ina second example, the system may include a solenoid valve disposed influid communication between the proportional control valve and the fluidoutput, where the solenoid valve is movable between an open position inwhich the proportional control valve is fluidly coupled to the fluidoutput, and a closed position in which the proportional control valve isnot fluidly coupled to the fluid output. In a third example, the systemmay include a controller; a sensor disposed in electrical communicationwith the controller, the sensor configured to detect the first andsecond brake pressures; wherein, the proportional control valve isoperably controlled by the controller to provide an output gain as afunction of the first and second brake pressures. In a further example,the system may include an operator display for communicating a valve setratio to the controller; wherein, a brake command is determined by thecontroller as a function of the valve set ratio, the first brakepressure, and the second brake pressure; further wherein, when neitherthe first brake or second brake produce any brake pressure, the outputof the proportional control valve is based on the valve set ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects of the present disclosure and the manner ofobtaining them will become more apparent and the disclosure itself willbe better understood by reference to the following description of theembodiments of the disclosure, taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a side view of a combined tractor and trailer system;

FIG. 2 is a control schematic view of a hydraulic system of a combinedtractor and trailer brake system;

FIG. 3 is a control schematic view of a combined hydraulic and pneumaticsystem of a combined tractor and trailer brake system;

FIG. 4 is a hydraulic control circuit of a combined tractor and trailerbrake system; and

FIG. 5 is a flow diagram of a control process for controlling a combinedtractor and trailer brake system.

Corresponding reference numerals are used to indicate correspondingparts throughout the several views.

DETAILED DESCRIPTION

The embodiments of the present disclosure described below are notintended to be exhaustive or to limit the disclosure to the preciseforms in the following detailed description. Rather, the embodiments arechosen and described so that others skilled in the art may appreciateand understand the principles and practices of the present disclosure.

Referring to FIG. 1, a combined tractor trailer system 100 is shown. Inthis system, a tractor 102 is shown coupled to and pulling a trailer104. The tractor 102 may have a chassis 106 that spans between a frontground engaging mechanisms 108 and a rear ground engaging mechanisms110. In the embodiment of FIG. 1, each ground-engaging mechanism is inthe form of a wheel defined along a respective axle, i.e., a front axleand a rear axle. In other embodiments, however, the ground-engagingmechanism may be a track that propels the tractor 102 along a groundsurface. Likewise, the trailer 104 may also include at least oneground-engaging mechanism such as a wheel 126.

A cab 112 may be coupled to the chassis 106 and define a location for anoperator to be positioned in an operator's seat 114. From the cab, theoperator may control the tractor 102 and trailer 104 via a plurality ofcontrols. As shown, the cab 112 may include a display 116 or dashboardthat visually shows control characteristics of the tractor 102 ortrailer 104 such as speed, power, temperature, pressure, direction, andany other type of control characteristic. The display 116 may be atouchscreen display that includes one or more operator controls forselectively controlling the operation of the tractor 102 or trailer 104.Other controls may include a steering wheel or yoke 118, a pedal 120(e.g., a brake pedal, clutch pedal, or throttle pedal), any other typeof control such as a joystick, switch, lever, knob, etc. for controllingthe tractor trailer system 100.

While a tractor 102 is shown and described herein, any type of workmachine may utilize the teachings of this disclosure and therefore it isnot intended to be limited to applying to only tractors. In otherembodiments, a truck configured to tow a trailer may utilize theteachings of this disclosure. Accordingly, the tractor 102 can be anytype of work machine used to pull a trailer.

The tractor 102 may be coupled to the trailer 104 through a hitch memberor drawbar 122. The hitch member or drawbar 122 may be sufficientlystrong to transfer motion of the tractor 102 to the trailer 104. In oneembodiment, when the tractor 102 travels in a forward direction, thehitch member 122 pulls the trailer 104 along therewith in approximatelythe same direction.

One or more fluid lines 124 may also be provided. For purposes of thisdisclosure, a fluid may include a gas or liquid. Thus, any pneumatic orhydraulic line may be referred to as a fluid line herein. The one ormore fluid lines 124 may selectively fluidly couple a tractor brakesystem to a trailer brake system. More specifically, the tractor 102 mayprovide a trailer brake output that is coupled to the trailer brakesystem. In this configuration, when the user initiates a brake command,both the tractor brake system and the trailer brake system maysimultaneously engage to slow the tractor 102 and trailer 104.

In some tractor trailer systems, one or more brake pedals may be engagedby an operator to apply a tractor service brake. Brake actuators may bepressurized to apply the brakes, which will be described in greaterdetail below with reference to FIG. 2. Once the operator commandsengagement of the tractor braking system, the trailer braking systemalso engages to prevent the trailer from contacting or running into aback end of the tractor. In many conventional braking systems, there maybe a single hydraulic line that connects between the tractor and trailerbraking systems. In addition, a single hydraulic trailer brake valve mayhave a fixed gain associated with it due to its geometry. Thus,depending upon the fixed gain, an amount of pressure entering the valveis multiplied by the fixed gain to deliver a predetermined outletpressure. As braking systems change and new federal and internationalguidelines or standards change, there is a need for both lower andhigher outlet pressures depending upon the type of trailer. As a result,an adjustable gain across the hydraulic trailer brake valve is desirableover a fixed gain. Other needs and advantages will become apparent fromthe principles and teachings of the present disclosure.

In some tractor trailer systems, the trailer braking system includes ahydraulic braking system, a pneumatic braking system, or a combinationthereof. An example of a hydraulic trailer brake system is shown in FIG.2. In this embodiment, a hydraulic braking system 200 of a tractor and atrailer may include a brake controller 202 as shown. The brakecontroller 202 may be in electrical communication with a vehiclecontroller (not shown), an engine controller (not shown), a transmissioncontroller (not shown), and any other type of controller for controllingthe tractor or trailer. The brake controller 202 may include a pluralityof inputs and outputs for receiving and communicating electrical signalsor commands to different components within the braking system 200. InFIG. 2, for example, the brake controller 202 may be disposed inelectrical communication with a tractor display 250. This communicationmay exist over a controller area network (CAN) bus 252 or communicationlink. The display 250 may allow an operator to selectively communicateinstructions to the brake controller 202 for controlling the tractorbraking system or the trailer braking system 200.

Other operator controls may also be in communication with the brakecontroller 202. For example, one or more brake pedals 204 may beselectively engaged by an operator. A brake position sensor 206 maydetect a movement of the one or more brake pedals 204 and communicatethis movement to the brake controller 202. Upon application of the oneor more brake pedals 204, a brake valve 212 may be triggered such thatfluid from a hydraulic supply line 254 (via a pump or other pressuresource) is delivered to brake actuators. In turn, the brake actuatorsare controllably actuated to deliver hydraulic brake pressure to thetractor braking system to control the speed of the tractor. In FIG. 2,the braking system 200 illustrates a right brake actuator 208 and a leftbrake actuator 210. In this embodiment therefore the operator maydepress a right brake pedal and a left brake pedal to engage the tractorbraking system. The right brake pedal and right brake actuator 208 maycontrol brake pressure to a service brake on the right, rear wheel ofthe tractor, and the left brake pedal and left brake actuator 210 maycontrol brake pressure to a service brake on the left, rear wheel of thetractor. In doing so, the right brake actuator 208 and left brakeactuator 210 deliver hydraulic pressure to the tractor service brakes toslow the vehicle. This hydraulic pressure may also be delivered to thetrailer braking system as will be described below.

As is known, an operator may apply the brake pedals to brake the tractorand slow its speed. In addition, an operator may gently apply or tap onthe brake pedal, as is often the case with an operator desiring todisable cruise control in a motor vehicle. In the tractor trailersystem, it may be desirable to begin braking the trailer as soon as thebrake controller 202 detects engagement of the one or more brake pedals204. In this example, the brake controller 202 may execute a pre-brakeroutine or algorithm to begin applying the trailer braking system. Inthe embodiment of FIG. 2, the brake pedal position sensor 206 can send asignal to the brake controller 202 indicating that the operator hasbegun to depress the pedals 204. Before any brake pressure or asubstantial amount of brake pressure is generated, the brake controller202 may execute the pre-brake routine and send a signal to the trailerbraking system to begin braking the trailer. This signal may be anadjustable or proportional signal from the brake controller 202. Thesignal may be received by a trailer brake valve that includes a firstsolenoid valve and a second 2-way position valve (i.e., an on/offvalve). In one example, up to seven bars of pressure may be output fromthe valve to the trailer brakes to begin a braking operation.

During this pre-brake routine, the first solenoid valve may receive thesignal from the brake controller and deliver the hydraulic pressure tothe trailer brakes. The second, 2-way position valve may be disposed inits open position to allow fluid pressure to pass through. However, ifthere is a failure detected in the system, the second, 2-way positionvalve can be actuated to its off or closed position to block fluidpressure to the trailer brake system. This “fail-silent” condition maybe utilized to protect against possible failures in the system. In anyevent, hydraulic pressure may still pass through a main hydraulictrailer brake valve 226 (FIG. 2) so that the trailer braking system isengaged during a braking operation. The pre-brake valve set, however,may be disabled or closed so that no output pressure passes through thevalve set to the trailer brakes.

In FIG. 2, hydraulic brake pressure from the right brake actuator 208may be detected by a first pressure transducer 214, and from the leftbrake actuator 210 may be detected by a second pressure transducer 216.The first and second transducers may be in electrical communication withthe brake controller 202. This electrical communication may be eitherwired or wireless communication, or any other known or to be developedform of communication. As a result, the brake controller 202 can monitorbrake pressure from both actuators.

A shuttle valve 218 may be disposed in fluid communication with theright brake actuator 208 and the left brake actuator 210. The shuttlevalve 218 may be actuated in either direction depending upon which brakepressure is the greatest. As such, the greater of the two brakepressures passes through the shuttle valve 218 and is referred to asbrake pilot pressure. In FIG. 2, the brake pilot pressure flowsdownstream from the shuttle valve 218 along a brake pilot line 242.

The braking system 200 of FIG. 2 may also include a hydraulic valve set220. This valve set 220 may be similar to the aforementioned pre-brakevalve set in that it includes both a proportional solenoid valve and a2-way shutoff valve. As shown, the brake controller 202 may be inelectrical communication with the hydraulic valve set 220 to control itsoperation. For example, a trailer brake command 248 may be communicatedfrom the brake controller 202 to the valve set 220. Moreover, the valveset 220 may send signals to the brake controller 202 in response to itsoperation (i.e., a pressure transducer 224 may communicate a pressureassociated with the valve set to the controller 202).

Outlet pressure from the hydraulic valve set 220 may flow through afluid line 244 as shown in FIG. 2. The hydraulic valve set fluid line244 and the brake pilot line 242 may converge upon a second shuttlevalve 222. Similar to the operation of the first shuttle valve 218, thegreater of the brake pilot pressure and hydraulic valve set pressure maypass through the second shuttle valve 222 and enter hydraulic line 246.The pressure in hydraulic line 246 may be referred to as the hydraulicpilot pressure. A pressure transducer 228 can detect the hydraulic pilotpressure and communicate this pressure to the brake controller 202, asshown in FIG. 2. This hydraulic pilot pressure may pass through ahydraulic trailer brake control line 230 to the trailer brakes.

In the system 200 of FIG. 2, the trailer may also include a trailer parkbrake. The trailer park brake may be operably controlled via a hydraulictrailer park control valve 232. This valve 232 may include a solenoid234 that is in electrical communication with the brake controller 202.The hydraulic trailer park control valve 232 may be biased to its ventedposition to allow pressure to flow through a supplementary hydraulicline 240. The valve 232 may also be in fluid communication with a tankor reservoir 236 as shown. As such, hydraulic pressure may be deliveredfrom a supply line 238 to the hydraulic trailer park control valve 232,and in its normally or biased open position, pressure passestherethrough to the hydraulic trailer brake supplementary line 240.

Referring to FIG. 3, a portion of the braking system 200 of FIG. 2 isremoved and an air or pneumatic trailer braking system 300 is shown. Asdescribed above, the trailer braking system may be hydraulic, pneumatic,or a combination thereof. In this disclosure, the pneumatic trailerbraking system may use any form of gas for controlling its brakes.However, for sake of simplicity, the type of gas will be referred to asair through the remainder of this disclosure. Thus, the pneumatictrailer braking system 300 will be referred to as the air trailerbraking system 300, but it is to be understood that any type of gas maybe used to control the trailer brakes.

In FIG. 3, the air trailer braking system 300 may include an air orpneumatic trailer brake valve 302. The air trailer brake valve 302 maybe located downstream from the second shuttle valve 222 of the hydraulicsystem 200 such that hydraulic pressure is used as an inlet to controlthe valve 302. Hydraulic pressure may flow through a pilot pressure line312 to an inlet of the air trailer brake valve 302.

In addition to the air trailer brake valve 302, the air trailer brakingsystem 300 may also include an air supply line 320, a shuttle valve 304,an air trailer park brake control valve (not shown), and a leak detectvalve 306. The shuttle valve 304 is disposed downstream of the airtrailer brake valve 302 and the air trailer park brake control valve.Thus, air pressure may be output from the air trailer brake valve 302via a first pressure line 314 and from the air trailer park brakecontrol valve via a second pressure line 316. The greater of the twopressures from the first and second line may trigger the shuttle valve304 open so that air pressure can flow through a first air trailer brakecontrol line 318 to the air trailer brakes. A pressure transducer 310may be in fluid communication with the control line 318 to detect thepressure and communicate it to the brake controller 202.

The leak detect valve 306 may be in the form of a normally open solenoidcontrol valve. The valve 306 may include a solenoid 308 that is disposedin electrical communication with the brake controller 202. As such, thebrake controller 202 can control the leak detect valve 306 between itsopen and closed positions via a communication link 324 with the solenoid308.

Air pressure may be supplied via the supply line 320 to the leak detectvalve 306. Since the leak detect valve 306 may be biased to its openposition, fluid may flow through the valve 306 to the air trailer brakevalve 302 and to an air trailer brake supply line 322.

The embodiments of FIGS. 2 and 3 are illustrative of only one example ofa braking system for a tractor trailer combination. Other embodiments ofa braking system is possible and within the scope of this disclosure.For instance, another embodiment may include one or moreelectrohydraulic secondary brake valves.

Referring to FIG. 4, a hydraulic trailer brake control circuit 400 isprovided for achieving adjustable gain and improved stability. As willbe described, an output of the hydraulic valve set 220 is combined withthe output of the hydraulic trailer brake valve 226, and the greater ofthe two outputs operably shifts a shuttle valve to provide hydraulicfluid to a trailer brake coupler. In other words, two independenthydraulic lines provide a hydraulic fluid to a shuttle valve, and thegreater of the two pressures is output to the hydraulic trailer brakecoupler.

As previously described, conventional trailer brake valves are designedwith a fixed gain such that an outlet pressure is a fixed ratio of theinlet pressure based on the gain. There is a need, however, for a lowerhydraulic trailer brake gain under new international standards. This maybe problematic with older trailers that have a low brake capacity,because a lower trailer brake valve gain can cause increased tractorbrake wear and poor braking control (i.e., tractor pushing). Thus, to beable to pull both newer and older trailers, an adjustable trailer brakesystem is desired such that the gain may be adjusted depending upon thetype of trailer being towed. Moreover, hydraulic trailer brake valvescan be unstable since they amplify pressure in the range of 3 to 17times the inlet pressure. Lower amplification valves tend to be morestable than higher amplification valves, and thus a control circuit suchas the one in FIG. 4 is desirable as it can utilize the lowest possibleamplification to satisfy certain standards and yet still be adjustableto a higher amplification for older trailers.

In the control circuit 400 of FIG. 4, a low amplification trailer brakevalve 408 may be provided in direct fluid communication with the brakepedals 204 and the right brake actuator 208 and the left brake actuator210. In particular, the trailer brake valve 408 may have a fixed gainthat is predefined to meet any regulation for low amplificationpressures. In this embodiment, a left brake pressure 402 and a rightbrake pressure 404 may be provided as the operator depresses the leftand right service brake pedals 204. The first shuttle valve 218 islocated upstream of the brake pedals 204 and actuators, which is similarto the embodiment depicted in FIG. 2. Depending upon which of the leftbrake pressure 402 and the right brake pressure 404 is greatest, theshuttle valve 218 may be actuated to either side to allow the greaterpressure to flow therethrough to the brake pilot line 242. In thismanner, a brake pilot pressure flows through the pilot line 242 in aninlet direction as shown by arrow 410.

The brake pilot pressure may then be multiplied by the fixed gain of thehydraulic trailer brake valve 408 to produce an outlet pressure thatflows through an outlet line 414 in a downstream direction 412. Theoutlet pressure flows through the outlet line 414 until it reaches ashuttle valve 416, as shown in FIG. 4. The operation of this shuttlevalve 414 will be described below.

The hydraulic valve set 220 of FIG. 4 is disposed in parallel relativeto the hydraulic trailer brake valve 408. The hydraulic valve set 220may take the form of an electrohydraulic valve set that is adjustable toproduce a higher amplification pressure for older trailers. Thehydraulic valve set 220 may include a first valve and a second valve.The first valve may be a pressure-controlled, proportional solenoidvalve 418 that may be adjustably controlled to produce a variable outletpressure. This valve may outlet between 0-35 bar of hydraulic pressure,although other embodiments may vary in the amount of outlet pressurefrom the valve. The brake controller 202, for example, may send acurrent to the control valve 418 to produce a desired pressure. Thecurrent may be varied so that the outlet pressure from the control valve418 is greater than the outlet pressure of the low amplification trailerbrake valve 408. The ratio across the valve set 220 may be operatorcontrolled via the display 250. For instance, the operator may set theratio to a defined value using the display 250, and this ratio may becommunicated to the brake controller 202. This ratio may be stored bythe controller 202 in a memory unit thereof. Unlike the fixed gain ofthe trailer brake valve 408, the operator may adjust the ratio acrossthe hydraulic valve set 220 to a value greater than the fixed gain orany desirable pressure value. For example, if the operator disconnects anewer trailer and connects an older trailer to the tractor, the operatormay adjust the valve set ratio accordingly.

The second valve may be a two-way or three-way, on/off solenoid valve420. The valve 420 may include a solenoid 422 that is in electricalcommunication with the brake controller 202. The valve 420 may be biasedin its open position to allow fluid to flow through the valve set 220,but if an issue arises similar to the “fail-safe” description above, thetwo-position valve 420 may be triggered via the brake controller 202 toits closed position, vent pressure to tank 236, and block flow throughthe valve set 220. In this event, brake pressure may still be fedthrough the low amplification hydraulic trailer brake valve 408 asdescribed above.

As also shown in FIG. 4, a pressure transducer 424 may be positioneddownstream from the valve set 220 to measure outlet pressure therefrom.This transducer 424 may correspond with transducer 224 of FIG. 2, whichis in electrical communication with the brake controller 202 tocontinuously communicate the outlet pressure to the controller.

The operation of the second shuttle valve 416 may be similar to thefirst shuttle valve 218. Here, outlet pressure in line 414 from thehydraulic trailer brake valve 408 and the outlet pressure from theelectrohydraulic valve set 220 in line 244 converge on the shuttle valve416. The greater of the two pressures will pass through the shuttlevalve 418 onto the hydraulic trailer brake control line 230.

Referring to FIG. 5, a process 500 is provided for determining theadjustable pressure that is output via the electrohydraulic valve set220. In this process 500, a plurality of blocks or steps may be executedin order to determine the brake command to the valve set 220 to achievea desired brake pressure. A first block 502, for example, may beexecuted by setting a valve set ratio. This may be done by the operatorinputting a gain value into a display 250 in the cab of the tractor. Indoing so, the operator may continuously adjust the valve set ratio asdesired. Alternatively, the brake controller 202 or other control unitmay define this value. In any event, the valve set ratio may be set oradjustably controlled in block 502.

Once block 502 is executed, the process 500 may advance to block 504 inwhich brake pressures are monitored by sensors. In FIG. 4, for example,a left brake pressure and a right brake pressure may be monitored by apair of transducers 214, 216. These transducers 214, 216 are inelectrical communication with the brake controller 202 so that in block506, as brake pressure is detected, the transducers may communicate thepressures to the controller 202.

As the controller 202 receives brake pressures from the transducers orsensors, it may execute block 508 by determining a brake command as afunction of the brake pressures and the valve set ratio. This may beachieved by taking an average of the brake pressures and multiplying theaverage by the valve set ratio. Alternatively, the controller 202 mayselect the maximum or minimum of the brake pressures and multiply thatpressure by the valve set ratio. In a further embodiment, the controller202 may be programmed to take a ratio of the pressures and multiply itby the valve set ratio. The manner in which the brake command isdetermined in block 508 may be stored in the controller and executed inthe form of an algorithm or routine. As such, the result of this blockmay be adjustably controlled by the operator by setting the valve setratio in block 502. It is understood that the brake command and valveset ratio may be in the form of a pressure, or may be computed orotherwise converted into a pressure command.

Once the brake command is determined in block 508, the controller 202may compare the brake command to the valve set ratio in block 510. Basedon the comparison, the controller 202 may select the greater of the twovalues in block 512. If the valve set ratio is the higher of the twovalues, then the amount of amplification via the electrohydraulic valveset 220 is determined by the operator. If, however, the brake command isgreater, then the process 500 may advance to block 514 where thecontroller 202 may determine an amount of current that is a function ofthe detected brake pressures. The current may be proportional to thedetected brake pressures. Alternatively, the current may be a factor orratio of the detected pressures. A look-up table, formula, graphicaldata, or other known means may be used by the controller 202 todetermine how much current to send to the proportional solenoid valve418. Once the amount of current to send is determined in block 514, thebrake controller 202 may communicate this current to the valve 418 inblock 516 and the pressure is amplified through the valve set 220 inblock 518 based on the amount of current sent by the controller 202.

In this embodiment, an operator can depress the brake pedals 204 andproduce a small amount of detectable brake pressure in block 506. As aresult, there may be a low or minimal amount of current sent to thevalve 418, or the valve set ratio set by the operator in block 502 maybe greater than the brake command such that the pressure in line 244 isbased on the valve set ratio set by the operator. In any event, if thereis brake pressure generated by the operator, then pressure will bereceived at both sides of the second shuttle valve 416.

Alternatively, the operator may slightly tap or depress the brake pedals418 without generating any detectable brake pressure in block 506. Inthis instance, there may be only the pre-brake pressure from theelectrohydraulic valve set 220, and this pre-brake pressure is whatpasses through the second shuttle valve 416 to the hydraulic trailerbrake control line 230. Thus, the trailer brakes may be pre-loaded evenif no brake pressure is detected in block 506 of the control process500.

While this disclosure has been described with respect to at least oneembodiment, the present disclosure can be further modified within thespirit and scope of this disclosure. This application is thereforeintended to cover any variations, uses, or adaptations of the disclosureusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this disclosure pertains andwhich fall within the limits of the appended claims.

1. A braking system of a work machine, comprising: a pressurized fluidsupply; a base valve fluidly coupled to the supply; a proportionalcontrol valve fluidly coupled to the supply; a fluid output configuredto be fluidly coupled to a braking system of a trailer; a first fluidpath selectively fluidly coupling the supply to the fluid output via thebase valve; a second fluid path selectively fluidly coupling the supplyto the fluid output via the proportional control valve; wherein, thebase valve comprises a fixed gain and the proportional control valvecomprises an adjustable gain.
 2. The braking system of claim 1, furthercomprising a shuttle valve fluidly coupled between the base valve, theproportional control valve, and the fluid output, where the shuttlevalve is movable in response to a difference between a first pressure inthe first flow path and a second pressure the second flow path.
 3. Thebraking system of claim 1, further comprising a solenoid valve disposedin fluid communication between the proportional control valve and thefluid output, where the solenoid valve is movable between an openposition in which the proportional control valve is fluidly coupled tothe fluid output, and a closed position in which the proportionalcontrol valve is not fluidly coupled to the fluid output.
 4. The brakingsystem of claim 3, further comprising: a controller; and a solenoid ofthe solenoid valve disposed in electrical communication with thecontroller, where movement of the solenoid valve between its open andclosed positions is controlled by the controller.
 5. The braking systemof claim 1, further comprising: a controller; a service brake valve influid communication with the base valve, the service brake valveconfigured to provide fluid upon actuation of a service brake of thework machine; a sensor disposed in electrical communication with thecontroller, the sensor configured to detect a brake pressure from theservice brake valve; wherein, the proportional control valve is operablycontrolled by the controller to provide an output pressure as a functionof the brake pressure.
 6. The braking system of claim 5, wherein thecontroller receives a valve set ratio and operably controls an output ofthe proportional control valve as a function of the brake pressure andvalve set ratio.
 7. The braking system of claim 5, wherein thecontroller operably controls the proportional control valve according toa variable current that is proportional to the brake pressure.
 8. Abraking system of a tractor trailer combination, comprising: apressurized fluid supply; a shuttle valve disposed in fluidcommunication with at least two brake pressures; a base valve fluidlycoupled to the supply, the base valve including an inlet and an outlet,where the inlet is fluidly coupled to the first shuttle valve forreceiving the greater of the first brake pressure and the second brakepressure; a proportional control valve fluidly coupled to the supply; afluid output configured to be fluidly coupled to a braking system of atrailer; a first fluid path selectively fluidly coupling the supply tothe fluid output via the base valve; and a second fluid path selectivelyfluidly coupling the supply to the fluid output via the proportionalcontrol valve; wherein, the base valve comprises a predefined, fixedgain and the proportional control valve comprises an adjustable gain. 9.The braking system of claim 8, further comprising a second shuttle valvefluidly coupled between the base valve, the proportional control valve,and the fluid output, where the shuttle valve is movable in response toa difference between a first pressure output by the base valve and asecond pressure output by the proportional control valve.
 10. Thebraking system of claim 8, further comprising a solenoid valve disposedin fluid communication between the proportional control valve and thefluid output, where the solenoid valve is movable between an openposition in which the proportional control valve is fluidly coupled tothe fluid output, and a closed position in which the proportionalcontrol valve is not fluidly coupled to the fluid output.
 11. Thebraking system of claim 8, further comprising: a controller; a sensordisposed in electrical communication with the controller, the sensorconfigured to detect the at least two brake pressures; wherein, theproportional control valve is operably controlled by the controller toprovide an output pressure as a function of the at least two brakepressures.
 12. The braking system of claim 11, further comprising: anoperator display for communicating a valve set ratio to the controller;a first brake and a second brake of the tractor, the first brakeproviding a first brake pressure of the at least two brake pressures andthe second brake providing a second brake pressure of the at least twobrake pressures.
 13. The braking system of claim 12, wherein a brakecommand is determined by the controller as a function of the valve setratio, the first brake pressure and the second brake pressure.
 14. Thebraking system of claim 13, wherein when neither the first brake orsecond brake produce any brake pressure, the output of the proportionalcontrol valve is based on the valve set ratio.
 15. A braking system of atractor trailer combination, comprising: a pressurized fluid supply; afirst brake and a second brake of the tractor, the first brake providinga first brake pressure and the second brake providing a second brakepressure; a shuttle valve disposed in fluid communication with the firstbrake pressure and the second brake pressure; a base valve fluidlycoupled to the supply, the base valve including an inlet and an outlet,where the inlet is fluidly coupled to the shuttle valve for receivingthe greater of the first brake pressure and the second brake pressure; aproportional control valve fluidly coupled to the supply; a fluid outputconfigured to be fluidly coupled to a braking system of a trailer; afirst fluid path selectively fluidly coupling the supply to the fluidoutput via the base valve; and a second fluid path selectively fluidlycoupling the supply to the fluid output via the proportional controlvalve; wherein, the base valve comprises a fixed gain and theproportional control valve comprises an adjustable gain.
 16. The brakingsystem of claim 15, further comprising a second shuttle valve fluidlycoupled between the base valve, the proportional control valve, and thefluid output, where the shuttle valve is movable in response to adifference between a first pressure output by the base valve and asecond pressure output by the proportional control valve.
 17. Thebraking system of claim 15, further comprising a solenoid valve disposedin fluid communication between the proportional control valve and thefluid output, where the solenoid valve is movable between an openposition in which the proportional control valve is fluidly coupled tothe fluid output, and a closed position in which the proportionalcontrol valve is not fluidly coupled to the fluid output.
 18. Thebraking system of claim 15, further comprising: a controller; a sensordisposed in electrical communication with the controller, the sensorconfigured to detect the first and second brake pressures; wherein, theproportional control valve is operably controlled by the controller toprovide an output pressure as a function of the first and second brakepressures.
 19. The braking system of claim 18, further comprising anoperator display for communicating a valve set ratio to the controller;wherein, a brake command is determined by the controller as a functionof the valve set ratio, the first brake pressure, and the second brakepressure.
 20. The braking system of claim 19, wherein, when neither thefirst brake or second brake produce any brake pressure, the output ofthe proportional control valve is based on the valve set ratio.